1. Field of the Invention
The present invention relates to a method and an instrument for measuring a focal variation in pattern exposure in a lithographic process involved in manufacture of semiconductor device or display devices such as liquid crystal display, and also to a method of fabricating a semiconductor device.
2. Description of the Related Art
With recent advancement in the degree of integration of semiconductor devices, patterns formed by lithography are becoming increasingly finer. The finer the patterns become, the smaller the required dimensional uniformities become. One major cause of impairing the dimensional uniformity is represented by focus error (focal variation) of a light exposure apparatus. Any focal variation may result in dimensional variation in the resultant pattern, and this is more distinct for an isolated pattern having only a small depth of focus, causing a considerable reduction in the pattern width. It is generally believed that fabrication of semiconductor devices of 90-nm generation needs focus management of as precise as ±50 nm or around.
An exemplary procedure for the focus management in a conventional light exposure apparatus is as follows. First, sample wafers are fabricated under various set values of the focus typically in the routine inspection, pattern width of the isolated pattern or the like is measured typically using a CD-SEM, and an optimum focus value is determined by preparing focus-CD curve. The obtained result is stored in the light exposure apparatus as an offset value, so as to enable the focus management.
Other proposals of specific examples of the in-line focus monitoring technique are as follows. Patent Document 1 proposes a method of measuring focal variation in which relations between angle of inclination of the resist pattern edge and focus position are obtained, and taper angle of the resist pattern is then calculated to thereby measure the focal variation. Patent Document 2 proposes a method of measuring a focus value based on variations in measured length in the longitudinal direction and thickness of a resist pattern using a specialized mark.
[Patent Document 1]
Japanese Patent Application Laid-open No. Hei 10-154647.
[Patent Document 2]
Japanese Patent Application Laid-open No. 2000-133569.
The focus management according to the conventional technique, however, needs a considerable length of time for the dimensional measurement of the pattern, and this makes it impossible to complete the management within a single day or a shorter time unit, and it is much less possible to monitor the focal variation within a time unit of several hours or still shorter time period. It is also generally believed that the focus management using a product wafer is not practical, because the general product processing does not change the focus within a single wafer or in a single product lot.
Another problem of the in-line focus monitoring technique proposed at present resides in that the detectable focal variation cannot satisfy a necessary level of precision, and that the measurement using the CD-SEM cannot improve reproducibility or accuracy in the measurement. This consequently makes it unpractical to carry out a precise focus monitoring.
The present invention is completed in order to solve the aforementioned problems, and an object thereof is to provide a simple and precise method for measuring focal variation, and is to finally provide a method of measuring a focal point capable of fabricating a semiconductor device in a stable manner through feed-back of information on the measured focal variation to the next product lot, and through feed-forward to the next process; an instrument used therefor; and a method of fabricating a semiconductor device.